Wet gas refers to natural gas wherein amounts of liquid hydrocarbons, water vapor, and free water are significantly higher than those required by pipeline conveying. Alternatively, wet gas can be simply defined as a gas which contains some liquid, wherein the amount of liquid can vary from a small amount of water or hydrocarbon to substantial amount of water and hydrocarbon. Generally wet gas is defined as gas/liquid system with a Lockhart-Martinelli parameter smaller than approximately 0.3. During the exploring of a gas field, the metering of wet gas is involved in individual processes from exploring single-well to integral-delivery of multi-well, purification treatment, and pressured delivery. Metering data of wet gas can facilitate to know about production ability and production conditions of gas wells, and is the primary basis for optimizing the production and improving managements to gas reservoir.
Currently, wet gas metering techniques in the art primarily include two sorts, wherein one sort is a separation metering method, and another sort is the online metering method without any separation.
The separation metering method is to utilize a separator to separate a wet gas fluid into a gas phase fluid and a liquid phase fluid, and then to respectively meter the flows of the gas phase and the liquid phase at respective outlet thereof. However, the separation effect of the separator on wet gas is poor, and due to the phenomena of liquid carry-over and gas carry-under, the error of the metering result is high. Secondly, the structure and the flow process of the separator are complex, and thus the maintenances and managements to the system are complex with many links which should be controlled. Thus, fee for maintaining the operation of the system is high, and the metering process is disadvantageous to realize automatic managements to production process.
The online metering method without any separation for wet gas can have two developing trends. The first developing trend relates to the use a single-phase gas flow meter (e.g., a Coriolis mass flow meter, a V-cone flow meter, an orifice plate flow meter, a turbine flow meter, a ultrasonic flow meter, a Venturi flow meter, a vortex flow meter, etc.) to meter wet gas, and is committed to study and develop various experiential models to find out “correction coefficients”, thereby to correct metered results, so that a so-called gas flow rate value can be obtained. In such wet gas metering method, metering means for a liquid flow rate generally includes a sampling method or a tracing method. However, the limitations of this method and challenges confronted thereby mainly include the following aspects:
(1) the scope for which the “correction coefficient” of the gas flow rate is applicable is limited and depends on the liquid flow rate, and however, the method for measuring the liquid flow rate decides that its precision is not high even poor; and
(2) the method utilizing the “correction coefficient” of the gas flow rate is limited to be used for wet gas having a very high gas content and a very low liquid phase content, and with the increase of liquid component content in the wet gas, the error of the method will become unacceptable.
The second developing trend relates to metering wet gas by following or modifying online metering technique of a multi-phase flow. FIG. 1 depicts an online metering scheme in the prior art, in which a differential pressure type flow measuring device 2 is used to measure the total flow differential pressure value of wet gas in pipeline 1; a phase fraction meter 3 arranged in the pipeline 1 is used to measure the gas void fraction of the wet gas in the pipeline 1; and a flow calculating module 4, based on the total flow differential pressure value and gas void fraction of the wet gas, can be used to calculate the gas volume flow rate Qg and the liquid volume flow rate Ql.
Since only one phase fraction meter is arranged in the pipeline 1, and the phase fraction meter per se involve a “drift” (e.g., the counting drift of a gamma-phase fraction meter), metering errors will be produced. Thus, precise metering to the gas void fraction of wet gas in a pipeline cannot be achieved.